Britain's most senior police officer has promised a "ruthless" investigation into the alleged conspiracy against former Tory Chief Whip Andrew Mitchell.

Met Police Commissioner Bernard Hogan-Howe defended his handling of the "plebgate" row after temporarily breaking off from his holiday to be briefed on progress.

Mr Mitchell has accused the police of deliberately trying to "toxify" the Conservative Party and destroy his career over the affair.

Mr Hogan-Howe has faced criticism of his handling of the case after saying that, despite the arrest of the officer, he had seen no evidence that "really affected the original account of the officers at the scene".

He pointed to his choice of Deputy Assistant Commissioner Patricia Gallan, head of Professional Standards, to head the investigation and the fact that it was being supervised by the Independent Police Complaints Commission.

He said: "The allegations in relation to this case are extremely serious. For the avoidance of doubt, I am determined there will be a ruthless search for the truth - no matter where the truth takes us."

Mr Mitchell was forced to quit amid a storm of protest - fuelled by the Police Federation - over claims he called officers "plebs" during an altercation in Downing Street.

Writing in the Sunday Times, Mr Mitchell said: "These awful toxic phrases which were hung round my neck for weeks and weeks in a sustained attempt to toxify the Conservative Party and destroy my career were completely and totally untrue."

He expressed incredulity at the latest developments in the case, which have led to the arrest of one officer from the diplomatic protection squad and another man from outside the force.

"If you had told me on September 19 (the day of the altercation) that the events revealed last week could take place in Britain today, I simply would not have believed you."

The Police Federation, which represents rank-and-file officers, also says it plans an independent review into "issues" with its operations.

Downing Street has insisted Prime Minister David Cameron "stood behind" his Cabinet colleague for as long as he could after criticism from some allies of Mr Mitchell.

Number 10 issued a statement after newspapers quoted members of the Mitchell camp claiming he had been left "swinging in the wind" by the premier who they say failed to act on CCTV evidence that cast doubt on the police account.

Meanwhile, former police minister Nick Herbert said he did not believe swearing at a police officer was sufficient grounds for a ministerial resignation.

"He apologised and the apology was accepted (by the officer). I don't think that was a resigning matter," he told Channel 4 News.

"The inquiry must take its course but they must get on with it because there is a very great danger, it seems to me, of public trust being undermined by this."

Mr Mitchell has admitted swearing at police who refused to let him ride on his bicycle through Downing Street - but has strenuously denied calling them "plebs".